Modern wireless communication increasingly uses more simultaneous receiver (Rx) and transmitter (Tx) communication links such as CDMA networks as well as simultaneous GPS and broadcast features. These Tx and Rx channels operate simultaneously with a large difference of levels. For example, when the Rx signal goes below noise, the Tx signal can be very high with the leaked Tx to Rx signal being also high even when a duplexer for attenuation is used. Furthermore, during communications, a number of interferers may be present at the Rx input. The combination of all these interferers, Tx and jammers, will result in an important Signal-to-Noise Ratio (SNR) degradation due to the limited linearity of the Rx channel such as cross-modulation noise, inter-modulation products noise, and the like.
Typically, regarding the Rx path, a Low-Noise Amplifier (LNA) is the first element to degrade the SNR and the performance is managed by its IIP3 specification. However, the mixer is also a non-linear block and the interference levels have to be managed to avoid further SNR degradation. Normally, an inter-stage filter is added between the LNA and the mixer to limit the impact of these interferers. The inter-stage filter is usually a non-integrated Surface Acoustic Wave (SAW) or Bulk Acoustic Wave (BAW) filter, which presents the disadvantages of cost and Bill-Of-Materials (BOM) increase. Additionally, two main factors limit the integration of such filters, namely, the resonance frequency sensitivity to temperature and process variations, and the Q-factor or quality-factor limitations (amount of resistance to resonance).
For example, to solve the above drawbacks of the prior art, various solutions of tracking the resonance frequency have been advanced. In one solution, namely, in U.S. Pat. No. 6,940,348 B1, the LC pass-band filter (a filter having an inductor L and a capacitor C) is matched with an auxiliary oscillator and using the locking loop, performs its control directly on the auxiliary oscillator and not on the LC filter itself. The control information is then applied on the LC filter in a second step. However, the LC filter is not included into any feedback control loop, which results in a number of drawbacks.
First, the matching between the LC filter and the oscillator's tank presents a certain degree of error or mismatch due to process and temperature gradients. Thus, even if the control applied on the oscillator itself is perfect, this same control cannot be true for the LC filter because the control loop cannot correct or eliminate the mismatch error between the LC filter and the tank's oscillator.
Secondly, a good matching between the LC filter and the oscillator's tank requires placing each one extremely close to each other. The oscillator's tank, being a strong source of electromagnetic radiation, is able to magnetically couple with the LC filter and considerably pollute the receiver input with many undesirable frequency interferers that will imply high degrading consequences on the receiver sensitivity.
Therefore, in view of these concerns and drawbacks, there is a continuing need for developing a new and improved system and method for controlling and tuning the resonance frequency of a filter which would avoid the disadvantages and above mentioned problems while being cost effective and simple to implement.